1. Field of the Invention
The present invention relates to a light source for broadband light modulation in an optical communication system that utilizes wavelength division multiplexing or time division multiplexing. Specifically, the present invention relates to a laser module that combines two laser beams with different wavelengths so as to cause a difference frequency effect in a nonlinear optical medium and thereby generate light having a longer wavelength than that of the two laser beams.
2. Description of the Related Art
A laser module that includes a distributed feedback type semiconductor laser (referred to as DFB laser in the following) or a distributed Bragg reflection type laser (referred to as DBR laser in the following) mounted in a package has been used as a single wavelength laser. For the DFB laser and the DBR laser, noise increases significantly with return light reflected by transmission lines or the like. Therefore, an optical isolator is inserted generally into the optical axis in the package to prevent the return light from reaching a semiconductor laser. However, such an isolator is expensive and becomes a primary factor in raising the cost of a single wavelength laser.
When two different wavelengths of light are incident on a medium having a secondary nonlinear optical effect, difference-frequency light as well as harmonic light and sum-frequency light are generated, and larger nonlinear light is produced by phase matching. A wavelength conversion device that takes advantage of this effect is known. FIG. 4 shows this kind of wavelength conversion device, which uses control light to convert the wavelength of signal light that represents information into another wavelength. For convenience, a wavelength conversion device that mainly utilizes a difference frequency generation effect will be described in the following.
The wavelength conversion device shown in FIG. 4 includes a coupler 63, an optical waveguide 62, and a splitter 64. The coupler 63 combines incident light A having a wavelength of xcex1 and incident light B having a wavelength of xcex2. The optical waveguide 62 is formed of a nonlinear optical material 61 and has a polarization inversion structure for phase matching. The splitter 64 separates difference-frequency light C. The incident light A of xcex1 and the incident light B of xcex2 are combined by the coupler 63 and enter the nonlinear optical waveguide 62. In the optical waveguide 62, difference-frequency light C having another wavelength [xcex3=1.24/(1.24/xcex1xe2x88x921.24/xcex2)] is generated by the nonlinear optical effect and then emitted from the optical waveguide 62, together with the incident light. The splitter 64 cuts off the light other than the difference-frequency light C.
For example, when xcex1 is 0.76 xcexcm and xcex2 is 1.49 xcexcm, the difference-frequency light C having a wavelength xcex3 of 1.55 xcexcm can be generated as signal light.
Among wavelength conversion devices that utilize the nonlinear optical effect, the highest efficiency of wavelength conversion can be achieved by an optical wavelength conversion device composed of a periodic polarization inversion layer in the shape of stripes that is formed on a LiTa(1xe2x88x92x)NbxO3 (0xe2x89xa6xxe2x89xa61) substrate and an optical waveguide. The conversion efficiency increases in proportion to the square of the power of the incident light. For example, when both the incident light A (0.76 xcexcm) and the incident light B (1.48 xcexcm) enter the optical waveguide at 10 mW, the power of the difference-frequency light C having a wavelength of 1.55 xcexcm is 0.4 mW. In contrast, when both the incident light A (0.76 xcexcm) and the incident light B (1.48 xcexcm) enter the optical waveguide at 50 mW, the power of the difference-frequency light C having a wavelength of 1.55 xcexc/m is raised significantly to 10 mW.
Such difference-frequency light is not converted into the original wavelength even if it is reflected by a reflection point of the transmission line and returns to the waveguide. Therefore, the noise of a laser (also referred to as LD in the following) is not increased, so that this module can eliminate the need for an optical isolator.
To introduce a semiconductor laser beam efficiently to the optical waveguide 62, however, a module having the configuration shown in FIG. 4 requires a long time for alignment with a lens. This leads to an increase in the number of optical components and makes mass production difficult. Therefore, even if the isolator is removed, the cost is not reduced.
Further, in the case of a laser module having the configuration shown in FIG. 4, laser beams emitted from a light source are collimated by a collimator lens, then multiplexed by the coupler 63, and directed into the optical waveguide 62 by a focusing lens. This configuration is very complicated, and the average coupling efficiency of two wavelengths is only 50% or less due to wavelength aberration of the focusing lens. Thus, even if the laser output is 150 mW, both wavelengths of light enter the waveguide only at 75 mW or less, resulting in the difference-frequency light of 22.5 mW at the most.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a single wavelength laser module that utilizes difference-frequency light, can provide sufficient optical output without an isolator, and can be mounted easily with a simple optical system.
A single wavelength laser module of the present invention includes the following: a first laser device for oscillating a laser beam having a first wavelength; a second laser device arranged parallel to the first laser device for oscillating a laser beam having a second wavelength; an optical waveguide device arranged next to the output ends of the first laser device and the second laser device; and an output optical fiber arranged next to the output end of the optical waveguide device. The optical waveguide includes a coupling waveguide region and an optical wavelength conversion region. The coupling waveguide region combines light having the first wavelength and the second wavelength into a single waveguide by being optically coupled directly to the first laser device and the second laser device. The optical wavelength conversion region includes an optical waveguide for generating difference-frequency light between the first wavelength and the second wavelength. The coupling waveguide region is coupled optically to the optical wavelength conversion region. The optical fiber is optically coupled directly to the optical waveguide of the optical waveguide device.
This configuration provides a simple structure and process that only require that the optical waveguide device is mounted before the fiber, and the two-wavelength LDs are mounted before the input waveguides of the optical waveguide device. Thus, laser beams can be introduced and combined with high efficiency in the nonlinear waveguide, and the resultant difference-frequency light can be guided easily to the optical fiber.
In the above configuration, it is preferable that the optical waveguide device has the function of converting a mode size so that the shape of a beam spot is close to the shape of the cross section of a coupling portion between the laser device and the optical waveguide or between the optical waveguide and the optical fiber. This makes it possible to perform coupling and wavelength conversion with even higher efficiency.
A single wavelength laser module having another configuration of the present invention includes the following: a two-wavelength laser array device that includes a first active stripe for oscillating a laser beam having a first wavelength and a second active stripe for oscillating a laser beam having a second wavelength, the first active stripe and the second active stripe being arranged in parallel with a space therebetween; an optical waveguide device arranged next to the output end of the two-wavelength laser array device; and an output optical fiber arranged next to the output end of the optical waveguide device. The optical waveguide device includes a pair of input waveguide ports, a coupling waveguide region, and an optical wavelength conversion region. The pair of input waveguide ports is optically coupled directly to each of the active stripes of the two-wavelength laser array device. The coupling waveguide region combines light having the first wavelength and the second wavelength into a single waveguide. The optical wavelength conversion region includes an optical waveguide for generating difference-frequency light between the first wavelength and the second wavelength. The coupling waveguide region is coupled optically to the optical wavelength conversion region. The optical fiber is optically coupled directly to the optical waveguide of the optical waveguide device.
This configuration further can facilitate a mounting process of the module because the two LDs are mounted on the waveguide at one time.
In the above configuration, it is preferable that a substrate of the two-wavelength laser array device is GaAs, the first active stripe is formed of a layer including GaInNAs, and the second active stripe is formed of a layer including AlGaInP. This makes it possible to produce an arrayed laser without degrading the characteristics of individual lasers.
A single wavelength laser module of yet another configuration of the present invention includes the following: a tandem two-wavelength laser device; an optical wavelength conversion device; and an output optical fiber. The tandem two-wavelength laser device includes a first active stripe for oscillating a laser beam having a first wavelength and a second active stripe for oscillating a laser beam having a second wavelength. The first active stripe and the second active stripe are arranged in series. The optical wavelength conversion device includes an optical waveguide that is optically coupled directly to the stripe of the tandem two-wavelength laser device and generates difference-frequency light between the first wavelength and the second wavelength. The optical fiber is optically coupled directly to the optical waveguide.
This configuration can eliminate a coupler, so that the size of optical components in the module can be reduced further and the module configuration can be made simpler.
In the above configuration, it is preferable that a substrate of the tandem two-wavelength laser device is GaAs, the first active stripe is formed of a layer including GaInNAs, and the second active stripe is formed of a layer including AlGaInP.